Mervin Weldon Rettenmund (born June 6, 1943) is an American former Major League Baseball player and coach. He played thirteen seasons with the Baltimore Orioles (1968-73), the Cincinnati Reds (1974-75), the San Diego Padres (1976-77) and the California Angels (1979-80).

He helped the Orioles win the 1969 and 1971 American League pennant, the 1970 World Series and the 1973 AL Eastern Division, the Reds win the 1975 World Series and the Angels win the 1979 AL Western Division. He also served as hitting coach for the 1989 World Series champion Oakland Athletics, as well as the Athletics' 1990 A.L. pennant-winners, and the 1998 National League champion Padres.

He finished 19th in voting for the 1971 AL MVP for playing in 141 Games and having 491 At Bats, 81 Runs, 156 Hits, 23 Doubles, 4 Triples, 11 Home Runs, 75 RBI, 15 Stolen Bases, 87 Walks, .318 Batting Average (which was third best in the American League to Bobby Murcer of the New York Yankees, and Tony Oliva of the Minnesota Twins ), .422 On-base percentage, .448 Slugging Percentage, 220 Total Bases, 4 Sacrifice Hits, 3 Sacrifice Flies and 2 Intentional Walks.

In 13 seasons he played in 1,023 Games and had 2,555 At Bats, 393 Runs, 693 Hits, 114 Doubles, 16 Triples, 66 Home Runs, 329 RBI, 68 Stolen Bases, 445 Walks, .271 Batting Average, .381 On-base percentage, .406 Slugging Percentage, 1,037 Total Bases, 36 Sacrifice Hits, 20 Sacrifice Flies and 15 Intentional Walks.

After his major league career, Rettenmund served as hitting coach for the Texas Rangers (1983-85), the Athletics (1989-90), the Padres (1991-99), the Atlanta Braves (2000-01), and the Detroit Tigers (2002).

After three years out of the majors, Rettenmund returned as hitting coach of the Padres in June, 2006, replacing Dave Magadan. However, he himself was replaced in mid-season the next year (July 31, 2007), by Wally Joyner

Rettenmund currently resides in San Diego, California.